The present invention relates to drop ejection devices such as ink jet printing devices and the like. More particularly, this invention relates to a method and apparatus for detecting volume variation between individual drop ejection devices.
Drop ejection devices are used to eject a wide range of fluids for a variety of different applications. For printing applications, drop ejection devices are used to eject a marking fluid such as ink onto a print media. Drop ejection devices are used in other applications as well, such as to eject modeling fluid in the case of three-dimensional modeling and to eject various medications in the case of medical delivery devices such as inhalers.
One such drop ejection device is a resistive heating device that is used to rapidly heat an aqueous fluid. The drop ejection device is activated by passing an electric current through the resistive heating device such as a resistor. In response to the electric current, the resistive heating device produces heat, that in turn, heats the aqueous fluid in a vaporization chamber adjacent the resistive heating device. Once the fluid reaches vaporization, a rapidly expanding vapor front forces fluid within the vaporization chamber through an adjacent orifice or nozzle. The vaporization chamber is replenished with fluid and the drop ejection device is ready to eject a second drop upon activation of the resistive heating device. Frequently, a plurality of drop ejection devices are formed with each drop ejection device capable of being activated individually.
U.S. Pat. No. 6,086,190, assigned to the assignee of the present invention, discloses a drop detecting technique for identifying defective drop ejection devices of a plurality of drop ejection devices. Drop ejection devices have various failure mechanisms. Permanent failures of individual nozzles can result from a defect in the heating element that prevents vaporization and drop ejection. Other permanent failures result from deposits forming within the vaporization chamber and on the heating element preventing proper transfer of heat from the heating element to the ejection fluid. Failures that are not permanent, such as the accumulation of dried ink on the nozzles require cleaning of the nozzles before ink can be ejected again. By determining nozzle failures, the printing system can take appropriate action to remedy the situation. In the case where one or more nozzle requires cleaning, this nozzle cleaning routine can be performed by the printing system to remedy this failure mode. In the case where a permanent failure has occurred in one or more individual drop ejection devices, the printing system can compensate for the individual nozzle that has failed.
The drop detection device disclosed in U.S. Pat. No. 6,086,190 includes a sensing element. The sensing element has an electric potential applied between the sensing element and the printhead. As drops are ejected from the printhead, charge is accumulated on the sensing element. Each drop of ink ejected from the printhead causes a spike or pulse of electric charge as these drops strike the sensing element. A sense amplifier produces an output signal in response to the electrical voltage imparted onto the sensing element by the ink drops.
There is an ever present need to accurately determine volumes of drops ejected from drop ejection devices. Accurate determination of drop volume is important for determining a volume of fluid ejected so that a volume of fluid remaining can accurately be determined. In addition, accurate determination of drop volume allows the drop ejection system to compensate for changes in drop volume over time. These drop volume determining devices should have low manufacturing costs so as to not add significantly to the cost of the drop ejection system. Finally, these drop volume determining devices should be capable of being relatively easily manufactured in a high volume manufacturing environment.
A device for determining drop volume for a drop ejection device is disclosed. The drop ejection device has a drop ejector and a reference drop ejector that exhibits less drop volume variation than the drop ejector. The drop ejection device includes a sensor responsive to impinging drops for producing a signal proportional to drop volume. The sensor is responsive to drops from the drop ejector to produce an electrical signal. The sensor is responsive to drops from the reference drop ejector to produce a reference electrical signal. Also included is a processing device responsive to the electrical signal and reference electrical signal for determining drop volume of the drop ejector relative to drop volume of the reference drop ejector.